What the set algebra returns when nothing remains, e.g. x & y for disjoint
regions or x - y when y covers x. It has no parts() or cells(),
contains no point, and is bounded. null_model() refuses one.
Examples
# Disjoint regions intersect in nothing:
nothing <- point_region(theta = c(1, 0, 0)) &
point_region(theta = c(0, 1, 0))
is_empty(nothing)
#> [1] TRUE
length(parts(nothing))
#> [1] 0
# And the algebra keeps going from there:
identical(
nothing | simplex_region(vertices = diag(3)),
simplex_region(vertices = diag(3))
)
#> [1] TRUE